DisplayGLX: add error checking for the dummy pbuffer creation BUG=angleproject:892 Change-Id: I7a15f2be19a598e40d4fa5a7c70d879cdbc6d32a Reviewed-on: https://chromium-review.googlesource.com/277011 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>